Given the task definition and input, reply with output. In this task, you will be presented with a context from an academic paper and a question separated with a 
. You have to answer the question based on the context.

Let all word vectors be normalized and $W$ be the word matrix. Inspired by BIBREF21 , where vector space models are used for evaluating topic coherence, we suggest to estimate the interpretability of $k$ th component as $ \operatorname{interp}_k W = \sum _{i,j=1}^N W_{i,k} W_{j,k} \left(W_i \cdot W_j \right). $  
 Question: How do they evaluate interpretability in this paper?

we suggest to estimate the interpretability of $k$ th component as $ \operatorname{interp}_k W = \sum _{i,j=1}^N W_{i,k} W_{j,k} \left(W_i \cdot W_j \right). $